1. Field of the Invention
The present invention relates to an exposure method and an apparatus therefor in pattern formation techniques for semiconductor elements, liquid crystal display elements, and the like.
2. Related Background Art
A photolithographic process, in which a circuit pattern such as a semiconductor element is formed, generally employs a method of transferring a pattern formed on a reticle (mask) onto a substrate such as a semiconductor wafer. A photoresist having photosensitive properties is applied to the surface of the substrate, and a circuit pattern is transferred .to the photoresist in accordance with an illumination light image, i.e., the shape of a transparent pattern of the reticle. In a projection exposure apparatus (e.g., a stepper), an image of the reticle pattern is focused/projected on the substrate (wafer) through a projection optical system.
In an apparatus of this type, illumination light is limited to an almost circular (rectangular) shape centered on the optical axis of an illumination optical system within a plane of the illumination optical system (to be referred as an illumination optical system pupil plane hereinafter) serving as a Fourier transform plane on a surface of a reticle on which a pattern exists, or within an adjacent plane, thus illuminating the reticle. For this reason, the illumination light is incident on the reticle at right angle. In addition, a circuit pattern is drawn on a reticle (a glass substrate constituting of quartz or the like) used in this apparatus. The circuit pattern is constituted by transmission portions (substrate bare surface portions), each having a transmittance of nearly 100% with respect to illumination light, and light-shielding portions (consisting of chromium or the like), each having a transmittance of nearly 0%.
The illumination light radiated on the reticle is diffracted by the reticle pattern, and a 0th-order diffracted light component and .+-.lst-order diffracted light components are generated by the pattern. These diffracted light components are focused by a projection optical system to form interference fringes, i.e., an image of the reticle pattern, on the wafer. An angle .theta. (reticle side) defined by the 0th-order diffracted light component and each of the .+-.1st-order diffracted light components are determined according to sin.theta.=.lambda./P, where the wavelength of exposure light is represented by .lambda. (.mu.m), and the pitch of the reticle pattern is P.
As the pattern pitch decreases, sin.theta. increases. In addition, when sin.theta. exceeds the reticle-side numerical aperture (NA) of the projection optical system, the .+-.1st-order diffracted light components are limited by the effective size of a plane of the projection optical system (to be referred to as a projection optical system pupil plane hereinafter) serving a Fourier plane of the reticle pattern and cannot pass through the projection optical system. That is, only the 0th-order diffracted light component reaches the wafer, and no interference fringes reticle pattern image) will be formed.
In the conventional apparatus described above, therefore, if the reticle constituted by only the transmission portions and the light-shielding portions is used, the fineness (minimum pattern pitch) P of a reticle pattern which can be formed on a wafer is given by P.perspectiveto..lambda./NA based on sin.theta.=NA.
Since the minimum pattern size is 1/2 the pitch P, the minimum pattern size is about 0.5.times..lambda./NA. In a practical photolithographic process, however, a given depth of focus is required due to wafer warping, the influence of a step formed in the wafer in the process, and the thickness of the photoresist itself. For this reason, a practical minimum resolution pattern size is represented by k.times..lambda./NA where k is the process coefficient which falls within the range of about 0.6 to about 0.8.
As will be appreciated from the above description, in the conventional exposure apparatus, in order to transfer a finer pattern, an exposure light source having a shorter wavelength or a projection optical system having a larger numerical aperture must be used.
It is, however, difficult to decrease the wavelength of the exposure light source (to a wavelength of 200 nm or less) at present because an appropriate optical material which can be used as a transmission optical member does not exist and a stable light source which can provide a large amount of light is not available. In addition, the numerical aperture of the state-of-the-art projection optical system is almost a theoretical limit, and hence an increase in numerical aperture is almost impossible. Even if the numerical aperture can be larger than that currently used, the depth of focus determined by .+-..lambda./NA.sup.2 abruptly decreases with an increase in numerical aperture. Consequently, the depth of focus used in practice further decreases, and the exposure apparatus cannot be used as a practical apparatus.
Under the circumstances, there has been proposed a phase shift reticle which has a phase shifter (a dielectric thin film or the like) for shifting the phase of light transmitted through a specific portion, of all the transmission portions of the circuit pattern of the reticle, from the phase of light transmitted through other transmission portions by .pi. (rad). Specifically, a spatial frequency modulation type phase shift reticle is disclosed in, e.g., Japanese Patent Publication No. 62-50811. The use of a phase shift reticle of this type allows transfer of a finer pattern. That is, the resolving power can be improved.
Various types of phase shift reticles have been proposed so far. Typical examples are of a spatial frequency modulation type, a shifter light-shielding type, and an edge emphasizing type. The differences in focusing characteristics between the spatial frequency modulation type phase shift reticle, the shifter light-shielding type phase shift reticle, the conventional reticle, and the edge emphasizing type phase shift reticle will be briefly described below with reference to FIGS. 10A to 13.
FIG. 10A is a view for explaining the arrangement of the spatial frequency type phase shift reticle. As shown in FIG. 10A, light-shielding patterns 12c are arranged on a reticle (glass substrate) 11A at a pitch P.sub.0, and a phase shifter is formed on one of two transmission portions sandwiching each light-shielding pattern 12c. Therefore, opening portions (pitch P.sub.0) other than the light-shielding patterns 12c are alternately constituted by transmission portions 12a and phase shift transmission portions 12b. With this arrangement, the amplitude of light transmitted through the reticle 11A becomes +1 (=exp(2.pi.i.times.0)) at each transmission portion 12a; and -1 (=exp(2.pi.i.times..pi.)) at each phase shift transmission portion 12b. That is, a pitch P.sub.A (the pitch of the transmission portions 12a) from +1 to +1 is twice the pitch P.sub.0 of the light-shielding patterns 12c. Hence, the spatial frequency (degree of modulation in the lateral direction in FIG. 10A) of the transmitted light is reduced to 1/2. That is, the pitch is doubled.
In this case, a lst-order diffracted light component is generated by such a pattern in a direction defined by sin.theta..sub.A =.lambda./P.sub.A, but no 0th-order diffracted light component is generated because light transmitted through each transmission portion 12c and light transmitted through each phase shift transmission portion 12b cancel each other. If, therefore, illumination light is incident on the reticle 11A at substantially a right angle, the light (diffracted light) propagates between a projection optical system 13A and a wafer 14A, as shown in FIG. 10B. That is, only .+-.lst-order diffracted light components D.sub.pA and D.sub.mA reach the wafer. As a result, interference fringes are formed on the wafer 14A by the +lst-order diffracted light component D.sub.pA and -lst-order diffracted light component D.sub.mA to produce a light intensity distribution EA, as shown in FIG. 10B. This light intensity distribution represents images of the patterns 12a to 12c on the reticle.
FIG. 11A is a view for explaining the arrangement of the shifter light-shielding type phase shift reticle. Referring to FIG. 11A, phase shift patterns 12b, each having a width equal to or larger than the resolving limit of a projection optical system 13B (FIG. 11B), are formed on a reticle (glass substrate) 11B at a pitch P.sub.B by patterning. Assume that the pitch P.sub.B is equal to the pitch P.sub.0 in FIG. 10A. In the shifter light-shielding type shown in FIG. 11A, the spatial frequency of the amplitude of transmitted light remains P.sub.B. Therefore, each lst-order diffracted light component is generated by this pattern in a direction .theta..sub.B defined by sin.theta..sub.B =.lambda./P.sub.B with respect to a 0th-order diffracted light component.
Note that the diffraction angle .theta..sub.B is almost twice the diffraction angle .theta..sub.A in FIG. 10A. If, therefore, the pattern pitch P.sub.B decreases, .+-.lst-order diffracted light components D.sub.pB and D.sub.mB cannot pass through the projection optical system 13B, as shown in FIG. 11B. In this case, since only a 0th-order diffracted light component D.sub.0B reaches a wafer 14B, interference fringes based on 0th- and .+-.1st-order diffracted light components are not formed, thus obtaining a flat light intensity distribution EB. That is, no reticle pattern images are formed.
In this case, in order to form pattern images on the wafer, sin.theta..sub.B &lt;NA.sub.R (where NA.sub.R is the reticle-side numerical aperture of the projection optical system 13B) must be satisfied, as described above. Therefore, the minimum pattern pitch P.sub.B allowing transfer of patterns in FIG. 11B is given by P.sub.B &gt;.lambda./NA.sub.R according to sin.theta..sub.B =.lambda./P.sub.B.
On the other hand, in the spatial frequency modulation type shown in FIG. 10A, since sin.theta..sub.A =.lambda./P.sub.A &lt;NA.sub.R, and P.sub.A =2P.sub.0 =2P.sub.B &gt;.lambda./NA.sub.R, the minimum pattern pitch P.sub.0 at which patterns can be transferred is represented by P.sub.0 &gt;.lambda./(2. NA.sub.R)=P.sub.B /2. Therefore, the spatial frequency modulation type phase shift reticle allows transfer of patterns at a pitch 1/2 that set in the shifter light-shielding type phase shift reticle.
FIG. 12A is a view for explaining the arrangement of the conventional reticle. Referring to FIG. 12A, light-shielding (chromium) patterns 12c are formed on a reticle (glass substrate) 11c at a pitch P.sub.c by patterning. Assume that the pitch P.sub.c is equal to the pitch P.sub.B in FIG. 11A. Hence, the spatial frequency of light transmitted through the reticle 11c (transmission portion 12a) is equal to that of the light transmitted through the shifter light-shielding type phase shift reticle shown in FIG. 11A. Consequently, a generation direction (diffraction angle) .theta..sub.c of each lst-order diffracted light component from this pattern is equal to the diffraction angle .theta..sub.B in FIG. 11B. Similar to the shifter light-shielding type, as the pattern pitch P.sub.c decreases, .+-.1st-order diffracted light components D.sub.pC and D.sub.mC cannot pass through a projection optical system 13C, and only a 0th-order diffracted light component D.sub.0c reaches a wafer 14C. As a result, a flat light intensity distribution EC is obtained, and no reticle pattern images are formed. That is, there is no difference between the shifter light-shielding type phase shift reticle and the conventional reticle in the minimum pattern pitch at which patterns can be transferred to a wafer.
FIG. 13 is a view for explaining the arrangement of the edge emphasizing type phase shift reticle. Referring to FIG. 13, light-shielding patterns 12c are formed on a reticle (glass substrate) 11D at a pitch P.sub.D by patterning, and a phase shifter 12b is formed only around each transmission portion 12a. Although a detailed description of the edge emphasizing type phase shift reticle will be omitted, an improvement in resolution cannot be theoretically expected, similar to the shifter light-shielding type phase shift reticle.
The use of the spatial frequency modulation type phase shift reticle allows formation of a finer pattern than that formed by the conventional reticle. However, it is difficult to manufacture the spatial frequency modulation type phase shift reticle. For example, since the conventional reticle is constituted by only light-shielding portions and transmission portions (reticle bare surface portions), the conventional reticle can be obtained by forming light-shielding portions on the glass substrate by one patterning operation. In contrast to this, the formation of the spatial frequency modulation type phase shift reticle requires at least a total of two patterning operations, including not only patterning for forming light-shielding portions but also patterning for forming phase shifters on transmission portions.
On the other hand, since the shifter light-shielding type phase shift reticle is constituted by only phase shift transmission portions and transmission portions (substrate bare surface portions), patterning is not required for forming light-shielding portions (each constituted by a metal film or the like). Therefore, the phase shift reticle of this type can be completed by forming phase shifters on the glass substrate by one patterning operation to form fine (a pattern width equal to or smaller than the resolving limit of the projection optical system) phase shift patterns on a circuit pattern portion (corresponding to the dark portion of a projected image on the wafer). However, as long as a general (conventional) illumination optical system is used, an improvement in resolution cannot be expected from the shifter light-shielding type reticle.
In addition, similar to the shifter light-shielding type, an improvement in resolution cannot be expected from the edge emphasizing type phase shift reticle. Furthermore, the phase shift reticle of this type demands accurate positioning between the light-shielding patterns and the phase shift patterns. It is, however, reported that positioning of these patterns in the edge emphasizing type can be performed in a self-aligned manner.